1. Field of the Invention
This invention relates to a process for producing a high density sintered body of metal or ceramic, and more particularly, relates to a process for changing a metal or ceramic porous body to a high density sintered body by hot isostatic pressing (HIP).
2. Description of the Prior Art
HIP process is used for producing a high density sintered body from a molded body obtained by molding metal or ceramic powder into a prescribed form or a pre-sintered body thereof by heating at a high temperature under a high pressure with argon gas or nitrogen gas. In the HIP process, a porous body such as the above molded body or pre-sintered body is placed in a pressure vessel, and pressured isotopically at 500 to 3000 atm by a high pressure gas with heating at 600 to 2500.degree. C. to obtain a densified body. As mentioned above, since a high pressure gas is used as a pressuring medium, it is necessary to take steps so the gas does not enter into the porous body. There are the following methods to prevent gas entering into the porous body. The pre-sintered body itself is made gas-impermeable by increasing the density up to more than 93 %, preferably more than 95%, of the true density. In the cases of usual gas-permeable molded bodies having a density of 40 to 75% of the true density and pre-sintered bodies having a density of less than 93 % of the true density, the molded body or pre-sintered body is placed in a gas-impermeable capsule, which is sealed, and densified by heating and compressing it from the outside of the capsule. In the above method, the capsule is previously prepared, and as a result, this method is difficult to be applied to a porous body having a complex shape.
Also, a powder layer is formed on the surface of the porous body, and converted to an impermeable membrane by softening the powder layer by heating to form a gas tight capsule. This method can be used for a porous body having a complex shape because of less restriction on the shape of a porous body. The compressing conditions with gas are disclosed in Japanese Patent KOKAI No. 54-146205 in the case of converting a powder layer formed on the surface of a porous body to an impermeable membrane, and similar compressing conditions with gas are disclosed in Japanese Patent KOKAI No. 54-144412 as to a silicon nitride porous body.
The powder layer has also been investigated as to the material thereof, and the powder layer disclosed in Japanese Patent KOKAI No. 59-35870 is formed on the surface of a silicon nitride porous body to yield a double layer structure. The inner layer is composed of a high melting point glass, a high melting point glass-forming material or a high melting point metal material, and the outer layer is composed of a low melting point glass or a low melting point glass-forming material capable of conversion to an impermeable membrane at a temperature lower than the material of the inner layer.
The powder layer disclosed in Japanese Patent KOKAI No. 59-116178 is formed on the surface of a ceramic porous body composed of a high silica porous glass or a nitride glass thereof.
The powder layer disclosed in DE 3403917C1 has a double layer structure of which the inner layer does not contain a sintering aid and the outer layer contains a sintering aid. By the above structure, the outer layer is imparted with a function to be converted to an impermeable membrane, and the inner layer is imparted with a function as a separation layer being sintered little in order to facilitate the removal of them after HIP treatment.
As the method of forming the powder layer, in general, powder is suspended in a solvent to form a slurry, and the slurry is applied onto a porous body by brushing, immersing, spraying or the like. The thickness of the powder layer is controlled by repeating the application and drying, or the like. Various methods are disclosed in the aforementioned Japanese Patent KOKAI Nos. 54-146205, 54-144412, 59-35870 and 59-116178 and DE 3403917C1. For example, in the process of DE 3403917C1, a reaction-bonded silicon nitride porous body containing 0 to 4 wt. % of Y.sub.2 O.sub.3 as a sintering aid and having a porosity of about 20 % is immersed in a slurry composed of 50 wt. % of Si.sub.3 N.sub.4 and 50 wt. % of isopropyl alcohol, and a powder layer of silicon nitride about 1 mm thick is formed on the surface of the porous body by the capillary action thereof. The porous body is dried at 110.degree. C. in a dryer to remove isopropyl alcohol, and a first layer is formed. Subsequently, the porous body is immersed in a slurry composed of 80 wt. % of Si.sub.3 N.sub.4, 15 wt. % of Y.sub.2 O.sub.3 and 5 wt. % of Al.sub.2 O.sub.3, and a second layer is formed on the first layer by the capillary action of the porous body. The porous body is dried again at 110.degree. C. in a dryer to remove isopropyl alcohol. Thus, the porous body obtained has a first layer of silicon nitride not containing a sintering aid and a second layer of silicon nitride containing sintering aid of Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3. The porous body is heated at 1820.degree. C. for 10 minutes in a nitrogen gas atmosphere to convert the surface powder layer to a gas-impermeable membrane. Subsequently, the porous body is treated with HIP at 1750.degree. C. at a pressure of 2000 Bar in an argon gas atmosphere. The densified body thus obtained is subjected to sandblasting, and the gas-impermeable membrane on the surface is removed to obtain a high density sintered body.
On the other hand, it is also known that the gas-impermeable membrane can be formed from a starting material which is not powder. In the production of a high density sintered body disclosed in Japanese Patent KOKAI No. 62-053162, a liquid inorganic polysilazane (--SiH.sub.2 NH--).sub.n is applied onto a metal or ceramic porous body to form a membrane, and the membrane is rendered high pressure gas-impermeable by the oxidation of the surface portion, the thermal decomposition of residual inorganic polysilazane and the softening of the oxidized surface layer, prior to sintering the porous body at a high temperature and a high pressure.
Incidentally, the impermeable membrane is compressed at an extremely high pressure of 500 to 3000 atm through the compressing medium gas during HIP treatment. The impermeable membrane is required to have a high reliability capable of keeping gas impermeability against such a high pressure. The conventional slurry coating method has a problem in reliability. That is, contraction of the powder layer occurs during drying, and cracks are liable to form. When the powder concentration of the slurry is raised in order to decrease the contraction, the adherence of the powder to the porous body is insufficient, resulting in layer separation and in unevenness of layer thickness. Though it is effective to add an organic binder in order to avoid the generation of contraction cracks, defects are liable to occur during the removal of the binder by thermal decomposition. Usual organic binders are polyethylene glycol, polyvinyl alcohol, polyvinyl butyral, methyl cellulose, polymethyl methacrylate, polybutylacrylate and the like, and they are decomposed by heating and evaporated. Therefore, when defects do not occur in the membrane through the removal process of the organic binder, the membrane is frequently separated in the temperature elevation process up to the softening temperature necessary for rendering the membrane impermeable.
While, the coating method of inorganic polysilazane disclosed in Japanese Patent KOKAI No. 62-053162 is excellent in the adherence to porous bodies, it has the following problems. Contraction of the membrane greatly occurs during thermal decomposition resulting in the generation of cracks in the membrane and in the separation of the membrane in small pieces together with a part of the porous body. Even if only the cracks generate without the occurrence of the above separation, it is usually difficult to remedy such by the softening treatment of the membrane. Besides, in the above process, the surface of an inorganic polysilazane is oxidized, and the oxidized portion is softened. However, since the softening temperature range is narrow, it is not easy to set the softening temperature for each porous body. Moreover, the softening point and viscosity of the membrane formed can be adjusted by mixing the liquid inorganic polysilazane with Si.sub.3 N.sub.4 powder, B.sub.2 O.sub.3 powder or Al.sub.2 O.sub.3 powder. The formation of the impermeable membrane depends on SiO.sub.2 produced by the oxidation of the inorganic polysilazane. However, the control of the oxidation degree is difficut, and the gas-impermeable membrane is often not formed.